We herein report an unusual case of profound brain infarction of the posterior circulation due to a dolichoectatic vertebrobasilar dissecting aneurysm (DVDA) originating from atherosclerosis. On autopsy, diffuse atherosclerosis was observed with a multi-fusiform aneurysm measuring 1 to 2 cm in diameter ranging from the left vertebral artery to the basilar artery. The microscopic findings of the aneurysm revealed severe stenosis of the artery caused by intimal thickening, intimal flap formation and thrombosis, indicating the presence of a dissecting aneurysm originating from atherosclerosis. The DVDA observed in this case was considered to be slowly progressive and associated with the development of atherosclerosis. The etiology of structural destabilization in patients with DVDA involves rupture of the internal elastic lamina, which is dislodged by massive hematomas that form atheromatous lesions.